This invention relates to an oxidation process, more particularly to a process for preparing terephthalic acid by oxidation of p-toluic acid or mixtures of p-toluic acid with p-xylene and/or with partially oxidized derivatives thereof, such as p-tolualdehyde.
Terephthalic acid is of great commercial importance as it is increasingly used as a starting material for the production of high molecular weight resins such as fiber- and film-forming polyesters.
The prior art teaches many processes for the liquid-phase oxidation of alkyl-substituted aromatic compounds to aromatic carboxylic acids. One of the first patents in this field is U.S. Pat. No. 2,245,528 in the name of Loder, which discloses a one-step process for oxidizing alkylaromatic compounds by molecular oxygen in the presence of a metal catalyst, a solvent such as acetic acid and optionally an oxidation initiator. However, even under severe conditions, the yield in dicarboxylic acids is low. For example, upon oxidizing a mixture of xylenes with air in acetic acid containing cobalt- and manganese acetates as catalysts at 185.degree.-200.degree. C. under a pressure of 50 atmospheres and in the presence of diethylketone as initiator, the yield in phthalic acids was only 2%, and the main reaction products were toluic acids together with other intermediate oxidation products.
A number of further patents disclose processes for the oxidation of p-xylene in one step with improved yields in terephthalic acid. These patents relate mainly to the use of specific activators such as bromine-containing compounds (U.S. Pat. No. 2,833,816), ketones (U.S. Pat. No. 2,853,514), or aldehydes (U.S. Pat. No. 3,036,122). Although some of these processes are applied commercially, they nevertheless suffer from serious drawbacks. For instance, severe corrosion problems arise when bromine-containing activators are used. When a ketone or an aldehyde is employed, part of it is inevitably lost, and the remaining portion is transformed mainly into acetic acid which must be recovered, purified, and commercialized for the process to be economically feasible. Nevertheless, despite those drawbacks, the use of an activator is considered as being an essential requirement for efficiently producing phthalic acids from xylenes.
In most cases, the use of a solvent is also claimed as being necessary. Low-molecular-weight fatty acids, more particularly acetic acid, are widely used for this purpose. The added amount of solvent must be sufficient in order to maintain the reactants and the reaction products in solution or at least in suspension without difficulty. In this way, the reaction mixture is easily agitated, the dispersion of oxygen is improved, the formation of by-products is minimized and the heat of reaction is easily removed by solvent evaporation. However, under the reaction conditions generally used, a significant amount of solvent is lost by co-oxidation. Moreover, the solvent must be separated from the other components of the reaction mixture, and then be purified and recycled. Obviously, this consumption of a part of the solvent and these operations to recover the remaining part result in additional processing costs.
In order to avoid the above-mentioned important problems, it has been suggested to carry out the oxidation of p-xylene in the absence of a solvent. In this case, it is obviously necessary to work at a temperature which is at least in the range of the melting point of p-toluic acid, i.e., about 180.degree. C., in order to obtain a liquid reaction mixture. The choice of temperature is therefore limited. Moreover, in the absence of a solvent, the handling of the reaction mixture as well as the separation and the purification of terephthalic acid are difficult. Generally, the reaction is carried out to the point where the content of terephthalic acid in the mixture does not exceed 60%, preferably 45%, by weight. Beyond this point, "it becomes difficult to handle the oxidation reaction mixture as a slurry, and hence, the operation of the reaction is adversely affected" (see U.S. Pat. No. 3,883,584).
In the absence of a solvent, the removal of the heat of reaction presents another difficulty in industrial scale production, as extensive fouling takes place in the reactor, even when terephthalic acid is not present in large amounts. Thus, in U.S. Pat. No. 2,696,499, which relates to the oxidation of xylene into toluic acids in the absence of a solvent, it is explained that "the essential design problem inherent in the cooling of the xylene oxidation mixture is the prevention of the deposition of solids".
U.S. Pat. No. 3,406,196 describes a two-stage process, wherein water is used as a suspension agent for terephthalic acid. In the first stage, an alkylaromatic compound, more particularly p-xylene, is oxidized by means of air in the absence of any additional solvent into partially oxidized compounds which, in the second stage, are further oxidized at a higher temperature in the presence of substantial amounts of water as a suspending medium. Bromine or a bromine-containing compound must be present to promote oxidation. Nevertheless, very high temperatures in the range of 200.degree. to 275.degree. C. and, more particularly, from 225.degree. C. to 250.degree. C. are required for achieving conversion of those partially oxidized compounds into terephthalic acid. Accordingly, the same or even worse corrosion problems are necessarily encountered than those which are present in processes wherein acetic acid is used as a solvent. Moreover, as stated in said patent, "appreciable losses of unreacted polyalkylaromatic compound by degradation and other side reactions tend to occur when such compounds are exposed to the higher temperatures found necessary for efficient conversion of the partial oxidation products, produced in the first stage of oxidation, to aromatic polycarboxylic acids". Clearly, the teaching of this patent is that the use of large amounts of water, even in the presence of a bromine promoter, does not give satisfactory results for oxidizing p-xylene into terephthalic acid in one step.
Actually, it has been known for a long time already that water is "a catalyst poison in oxidation reaction" (U.S. Pat. No. 2,696,499). According to the most widespread opinion, water has an adverse effect upon the reaction rate by interferring with initiation. As a general rule, its presence is avoided as much as possible, regardless of whether a solvent and/or an activator are present. Thus, U.S. Pat. No. 3,064,044 describes an improved technique for maintaining a final (bromine-promoted) oxidation under substantially anhydrous conditions. In U.S. Pat. No. 3,519,684, which relates to an oxidation process wherein peracetic acid is used as promoter, it is specified that "preferably, nearly anhydrous conditions are employed, although a water content of up to about 10% can be tolerated and a maximum water content of not greater than 5% is preferred". In a continuous process for the oxidation of xylenes in the absence of any promoter and wherein the partially oxidized intermediates are continuously recycled, water is removed from the liquid effluent before recycling the latter in order to retain "the water content in the reaction mixture at less than 15% and preferably less than 5% of the total reaction mixture" (U.S. Pat. No. 3,700,731).
More recently, it has been found that, unexpectedly, the oxidation of p-xylene into terephthalic acid can be carried out in the presence of substantial amounts of water as a solvent, although in the absence of any brominated activator which earlier was considered as an essential requirement. This process is described in the co-pending U.S. Patent Applications Ser. No. 764,981 and Ser. No. 785,827. They comprise oxidizing p-xylene in the liquid phase by an oxygen-containing gas in the presence of p-toluic acid, water and of a heavy metal salt as a catalyst at a temperature of from about 140.degree. C. to about 220.degree. C. under a pressure sufficient to maintain at least a part of the water in the liquid phase. However, as the mutual solubility of p-xylene and water is low at the working temperature, such mixtures of water, p-xylene and p-toluic acid may separate into two phases: an aqueous phase and an organic phase which is rich in hydrocarbon and also contains an important portion of the p-toluic acid which is present in the reaction mixture. In this case, the oxidation reaction takes place mainly in the organic phase where the concentration of water is relatively low. Therefore, the desired solvent effect of water is partly lost. Moreover, this phase separation causes important technical difficulties with regard to homogenization, oxygen dispersion and mass transfer effects.